The helm used to drop from Onyxia back at lvl 60 but when Blizzard revamped the encounter for level 80s, the helm was moved to Nefarian's loot table. However, the helm dropping from Onyxia does have the same coloring than the actual T2 helm, so if a person just wishes to complete the full looks, they can get the respective helm from 10 or 25 man Onyxia as well.

Some suggestions for Cloaks: (similar or identical models are grouped in { } brackets, the alternate models are also in the item comparison link but you have to equipthem to the set and view them manually):